There has been much research and development on the photoelectric effect of inorganic and organic semiconductors. This effect is utilized in a variety of fields. Photoconductive and photovoltaic effects are well known and utilized in a variety of applications.
One example of a photoconductive material is cadmium sulfide. As shown in FIG. 5 taken from "Electronics Physics Engineering" by Masaharu Aoki, page 360, when a cadmium sulfide crystal is irradiated with light, the energy of the light generates a free electron and a free hole. When the electron and the hole pass through the crystal to the electrodes or to the electrode and from the crystal, the crystal is photoconductive.
A well known example of a photovoltaic element is a photocell including a semiconductor pn junction for detecting light. FIG. 6 illustrates this light responsive process in a semiconductor photodiode as described on page 304 of "Foundation of Opto-electronics" by A. Yariv, translated by Kunio Tada and Takeshi Kamiya. In FIG. 6, at a region A, an incident photon is absorbed at the p-type region, and a hole and an electron are generated. If the distance between the location where the hole and electron are generated and the depletion layer is less than an electron diffusion length, the electron reaches the depletion layer, drifts under the influence of the electric field, and traverses the depletion layer. At a region C, when the photon is absorbed close to the depletion layer in the n-type region, a hole generated in response diffuses toward the depletion layer and drifts through the depletion layer. A photon may also be absorbed in the depletion layer at a region B. Then, the generated hole and electron both drift under the influence of the electric field in opposite directions so that they reach the p-type side and n-type side, respectively.
The photovoltaic effect occurs in organic compounds as if a Schottky barrier were produced between a metal electrode and the organic compound. The organic compound layer generates carriers in response to the light absorption. An example is disclosed in detail in chapter 4 of D. A. Seanor's "Electrical Properties of Polymers".
Conventional photoresponsive elements, especially utilizing photoconductive and photovoltaic effects, are constituted as described above. The photoconductive property is utilized in a photosensitive plate or an optical sensor in photocopiers and the photovoltaic effect is utilized in solar cells or photosensors. The semiconductor pn junction can be produced in an MOS structure and fine patterning is possible. Thereby, a CCD image sensor with more than one million pixels has been made.
The high frequency response characteristics are important in a photoresponsive element because high sensitivity and high speed response are desired. In a conventional photoconductive element, the band structure of a semiconductor is exploited, and, in a photovoltaic effect element, a semiconductor pn junction or a metal semiconductor Schottky barrier is utilized. Since the diffusion time of carriers generated in the p-type or n-type region is finite and the width of the space charge layer at the pn junction or the Schottky barrier is finite, a delay occurs in response to incident light due to the transit time of the charge carriers. Therefore, in order to enhance the speed of response in conventional elements, it is necessary to shorten the time for the carrier transit by reducing the width of the space charge layer or the semiconductor layer. However, when the element is thin, the number of charge carriers produced and the sensitivity are reduced.